An annular, flexible, laminated bearing is disposed about a concentric axis and has an outer edge secured to a first structure and an inner edge encircling and secured to a second structure which normally lies along the bearing axis. The second structure can be rotated fractionally with respect to the first structure about an axis of rotation intersecting the bearing axis, somewhat like a clapper rotates with respect to the mouth of a bell. Such bearings typically are constructed of laminated alternate layers of an elastomer having a low shear modulus and a rigid material such as machined steel. Each lamina is disposed on the surface of a sphere centered at the intersection of the bearing axis and the axis of rotation, or on a conic section or cylinder centered on the intersection. In such a structure, individual lamina can accommodate fractional rotation of the second structure by shifting in their plane (or more precisely, along the surface of the cylinder, sphere or conic section on which they lie), while accommodating very little movement in a direction normal to the lamina (tending to pull the lamina apart or push them together). The following patents discuss such bearings and their construction and use: U.S. Pat. No. 3,429,622, issued to Lee et al on Feb. 25, 1969; U.S. Pat. No. 3,941,433, issued to Dolling et al on Mar. 2, 1976; and U.S. Pat. No. 4,395,143, issued to Bakken et al on Jul. 26, 1983. Those patents are hereby incorporated herein by reference to show the utility, structure, and methods of fabricating flexible bearings. Such bearings are used in rocket motors to secure a thrust nozzle to a relatively fixed casing so the nozzle can be steered to direct thrust either parallel to the longitudinal axis of the rocket body or along a skewed axis. Such laminated bearings also find many other uses, for example, for supporting a skewable drive shaft.
The most common material for the elastomeric components of such bearings is a low shear modulus, Hevea (natural) rubber formulation containing low levels of sulfur and fillers. Hevea rubber or its closest synthetic substitutes are essentially pure cis-1,4-polyisoprene (98%), and contain little or no 1,2-polyisoprene, 3,4-polyisoprene or trans-1,4-polyisoprene. An important characteristic of compositions consisting almost entirely of cis-1,4-polyisoprene is their tendency to crystallize. Cis-1,4-polyisoprene exhibits strain crystallinity when it is under stress, which gives it highly desirable physical properties including a high shear strength for a low shear modulus composition.
Unfortunately, Hevea rubber or other high cis-1,4-polyisoprene compositions are also susceptible to thermal crystallization when maintained substantially below room temperature. Thermally crystallized cis-1,4-polyisoprene does not have the desirable low shear modulus necessary to flex when a minimal load is exerted against the thrust nozzle. Kirk-Othmer's Encyclopedia of Chemical Technology (3rd Edition), Vol. 20, page 472, indicates that Hevea rubber thermally crystallizes at an appreciable rate below about 20.degree. Celsius, with the rate depending on the temperature. At 8.degree. C., crystallization will be evident after about one month, while at minus 26.degree. C. Hevea rubber hardens in a few hours. Thus, when a rocket motor or other structure is stored at a cold ambient temperature, the rubber composition may crystallize so much that the flexible bearing will no longer function properly until its temperature increases substantially.
The other type of natural (and corresponding synthetic) rubber which is capable of crystallizing is trans-1,4-polyisoprene, the naturally derived manifestations of which are balata and gutta percha. This isomer is highly crystalline, thermoplastic, and hard.
The problem of thermal crystallization of cis-1,4-polyisoprene cannot be solved by changing the proportions of non-rubber additives. If more sulfur is added to the composition, the greater amount of cross-linking upon vulcanization tends to prevent the development of thermal crystallinity at low temperatures, but also increases the shear modulus of the rubber composition to an unacceptable level for use in laminated bearings.
Andrews, et al., "Microkinetics of Lamellar Crystallization in a Long Chain Polymer", Rubber Chemistry and Technology, Vol. 45, pp 1315-1333 (1972), demonstrates that, for unvulcanized compositions, the isomerization of essentially pure cis-1,4-polyisoprene to provide a copolymer containing trans-1,4-polyisoprene slows down the rate of thermal crystallization of the composition greatly. But that reference employs unvulcanized compositions, which are unsuitable for making flexible, laminated bearings, to show the effect of isomerization on crystal growth rates as a function of temperature.
Mixtures and copolymers of cis-1,4-polyisoprene and trans-1,4-polyisoprene are known. Besides the Andrews article, the most pertinent references are believed to be U.S. Pat. No. 3,817,954, issued to Kawakami et al. on Jun. 18, 1974 (see Tables 4, 7, and 9); and U.S. Pat. No. 2,363,654, issued to Daly (page 1, column 2). The Kawakami et al. reference can be distinguished because it suggests materials having a substantially higher modulus than is useful here. The Daly patent can be distinguished because it indicates on page 1, from column 1, line 46 to column 2, line 7, that the product produced is hard and thermoplastic, unlike the present, low shear modulus compositions. The isomerized rubber of Daly evidently is a copolymer of cis-1,4-polyisoprene and trans-1,4-polyisoprene moleties. Many such compositions are distinguished by the presence of a substantial proportion of 1,2-polyisoprene or 3,4-polyisoprene, which degrade the physical properties of the composition.
Several other references, believed to be less pertinent than the foregoing, are as follows:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ 3,060,989 Railsback, et al. 10/30/62 3,488,341 Fischer et al. 01/06/70 3,661,883 Nishida, et al. 05/09/72 3,676,416 Makimoto 07/11/72 3,687,925 Fukui 08/29/72 4,035,444 Yang 07/12/77 4,130,606 Van Ballegooijen, et al. 12/19/78 4,385,151 Furukawa, et al. 05/24/83 4,414,363 Akita, et al. 11/08/83 4,430,487 Sandstrom 02/07/84 4,433,107 Takeuchi, et al. 02/21/84 4,461,883 Takeuchi, et al. 07/24/84 4,521,587 Furukawa, et al. 06/04/85 ______________________________________
Kirk-Othmer, Encyclopedia of Chemical Technology, 3d Ed., Vol. 13, page 827 (1981).
Particular elastomeric bearing compositions taught in the prior art are found in the previously cited Bakken et al. (column 2) and Dolling et al. (column 2) references.